This invention relates to surface treatment of a metal substrate, particularly aluminum, prior to application of an adhesive to the substrate, and is particularly directed to a process for the treatment of a metal substrate such as aluminum, to enhance subsequent adhesive bonding, by means of a chemical treatment which permits a controlled etching of the substrate surface and a uniform increase in surface area.
The adhesion of polymers (as adhesives) to metal substrates is, generally, a complex process. Usually, the metal surface has to be pretreated, and the adhesive has to have sufficient activity for the mating of the two to result in an optimum bond. Substrates, e.g. metals, have to be prepared for this union of such two dissimilar materials as a polymer and a metal by first cleaning and then etching the metal, and, as in the case of aluminum, by anodizing (oxidizing) or other surface treatment such as the so-called FPL etch, employing a solution of sodium dichromate in sulfuric acid. These treatments have the effect of increasing surface area in order to enhance mechanical interaction between the polymer and the metal substrate. Where mechanical interactions alone between the metal and polymer coating are involved, due primarily to surface roughening, the resulting bond strength is relatively weak.
Thus, surface treatment of metal substrates for subsequent application to adhesive bonding or painting has become a very important problem. With the advent of controlled oxidation techniques, i.e. anodization or other oxidative etching techniques, such as the well-known FPL etch noted above, or phosphoric acid anodization, among others, surface preparation has become a relatively simple matter. However, when bonding or coating on such a prepared surface, the majority of the methods take advantage of physical or mechanical bonding to the substrate, resulting in a relatively weak bond between the metal and polymer adhesive, as opposed to having a true chemical bond between the substrate and the adhesive.
U.S. Pat. No. 4,180,442 discloses electrodeposition of a coating of an organic compound or polymer on a metal such as aluminum. The polymer coating has a strong chemical bond to the metal substrate, and also has functional groups for bonding to an adhesive, enabling a total chemical bond to be formed via such coating between the metal substrate and the adhesive. However, lap shear values measuring the strength of the adhesive bond to such coated substrates have varied from as high as over 5300 psi to as low as about 3000 psi. Thus, in order to obtain more consistently high lap shear values, it became evident that surface roughness or high surface area is highly important.
In adhesive bonding to a metal, such as aluminum, it is usually necessary to pretreat the metal surface by cleaning it with a degreaser, followed by an alkaline clean and etch, and then by a deoxidizing process, followed by either anodizing it or subjecting it to a surface treatment such as FPL etch. Subsequently, these surfaces are normally treated with a primer to effect a mechanical bond between the substrate and the primer, and then an adhesive is applied to the primer. In the process of the above patent one proceeds from the deoxidized surface directly to the electrodeposition of the organic coating without passing through the surface preparation technique of oxidation. It is because of this that in the practice of the process of the above patent, electrodeposition takes place principally on a surface of variable roughness, thereby resulting in variable values in the lap shear bond tests.
Since the electrodeposition of a coating procedure according to the above patent does not utilize an oxidized, e.g. anodized surface as a precursor to the electrodeposition step, the surface of the substrate is only as rough as it is when received from the mills. The only pretreatment of the substrate normally is to subject the substrate surface to a cleaning and a deoxidizing process, as noted above.
Thus, to provide chemical bonding from the metal through to the adhesive, as opposed to the physical bond formed with oxides, a chemical treatment is needed that will etch the surface, as well as deposit a chemical compound that will function as an "active" surface for the electrodeposition process. This, then, would create a high surface area substrate that has a chemical compound attached chemically, and that will, in turn, chemically bond to the adhesive.
Numerous methods are known for increasing the surface area of a substrate, and roughening a surface, such as by sanding, for example, but it is difficult to perform in a production line system. Furthermore, it does not create a uniform surface. Aside from the oxidation methods described above, there are very few other techniques for increasing surface area in a uniform manner.
Generally, in a galvanic cell, one element is present as an ionic species and the other is metal. Then, if the electromotive force of the cell is such that one part of the cell has a lower reduction potential than the other, the reducing element will reduce the ion in solution to the metallic state and the reducing element will become oxidized and go into solution. Thus, it is well known that metallic zinc will reduce copper ions to copper metal, and the zinc metal will go into solution. Similarly, metallic aluminum will reduce copper ions to copper metal, and the aluminum will go into solution. In the case of a sheet of aluminum (or its alloy), in a copper salt solution, there will result a severely pitted aluminum sheet with large masses of copper attached to its surface, and this action will continue as long as there are copper ions in the solution.
Accordingly an object of the present invention is the treatment of a metal surface such as aluminum by a chemical procedure to enhance adhesive bonding to the metal. Another object is the provision of procedure for chemically etching the surface of a metal such as aluminum so as to uniformly attack the surface of the metal and to increase the surface area of the metal in a uniform manner. A further object is to provide a chemical treatment for metals such as aluminum, which will etch the surface uniformly, and also deposit a chemical compound which will also function as an "active" surface for the electrodeposition process of the type described in the above patent. Yet another object is a chemical treatment procedure for a substrate such as aluminum which creates a high surface area substrate having a chemical compound attached chemically thereto which will in turn chemically bond to an adhesive. A still further object is the provision of a chemical treatment procedure of the type noted above which uniformly attacks the surface of a substrate and provides a high surface area, in conjunction with electrodeposition of a coating to provide a chemical bond for an adhesive coating resulting in more consistently high lap shear values in adhesive bonding tests.